Satellites circling the earth will employ Brayton or Stirling engine cycles as a power source that requires a heat storage system to power the engine. The storage system collects heat when the satellite passes between the Earth and the sun and utilizes the stored heat during the eclipse phase of its orbit. Generally, this process employs a heat storage material that melts when it absorbs heat and solidifies as it releases heat to a gas that circulates contiguous to the heat storage material. It is the heated gas that powers the engine.
It is important that the heat storage system be capable of operating at optimum efficiency during the eclipse phase of the orbit. The conventional approach to this thermal storage problem is to use the latent heat of fluoride salts. The fluoride salt, contained within a superalloy canister, melts during insolation and freezes during the eclipse. Although candidate fluorides have large heats of fusion per unit mass, their poor thermal conductivity limits the rate at which energy can be transferred to and from the storage device. System performance is further limited by the high parasitic mass, the mass that does not directly contribute to heat storage, of the superalloy canisters needed to contain the salt. Also, the large volume change when the fluoride freezes leads to a large void volume in the solid state, further interfering with heat transfer.
In view of these limitations there is a continuing need to develop heat storage systems for satellite applications that have a low parasitic mass and good thermal conductivity characteristics.